DESCRIPTION

Given a
pathname
for a file,
open()
returns a file descriptor, a small, nonnegative integer
for use in subsequent system calls
(read(2), write(2), lseek(2), fcntl(2), etc.).
The file descriptor returned by a successful call will be
the lowest-numbered file descriptor not currently open for the process.

By default, the new file descriptor is set to remain open across an
execve(2)
(i.e., the
FD_CLOEXEC
file descriptor flag described in
fcntl(2)
is initially disabled); the
O_CLOEXEC
flag, described below, can be used to change this default.
The file offset is set to the beginning of the file (see
lseek(2)).

A call to
open()
creates a new
open file description,
an entry in the system-wide table of open files.
The open file description records the file offset and the file status flags
(see below).
A file descriptor is a reference to an open file description;
this reference is unaffected if
pathname
is subsequently removed or modified to refer to a different file.
For further details on open file descriptions, see NOTES.

The argument
flags
must include one of the following
access modes:
O_RDONLY, O_WRONLY, or O_RDWR.
These request opening the file read-only, write-only, or read/write,
respectively.

In addition, zero or more file creation flags and file status flags
can be
bitwise-or'd
in
flags.
The
file creation flags
are
O_CLOEXEC,
O_CREAT,
O_DIRECTORY,
O_EXCL,
O_NOCTTY,
O_NOFOLLOW,
O_TMPFILE,
and
O_TRUNC.
The
file status flags
are all of the remaining flags listed below.
The distinction between these two groups of flags is that
the file creation flags affect the semantics of the open operation itself,
while the file status flags affect the semantics of subsequent I/O operations.
The file status flags can be retrieved and (in some cases)
modified; see
fcntl(2)
for details.

The full list of file creation flags and file status flags is as follows:

O_APPEND

The file is opened in append mode.
Before each
write(2),
the file offset is positioned at the end of the file,
as if with
lseek(2).
The modification of the file offset and the write operation
are performed as a single atomic step.

O_APPEND
may lead to corrupted files on NFS filesystems if more than one process
appends data to a file at once.
This is because NFS does not support
appending to a file, so the client kernel has to simulate it, which
can't be done without a race condition.

O_ASYNC

Enable signal-driven I/O:
generate a signal
(SIGIO
by default, but this can be changed via
fcntl(2))
when input or output becomes possible on this file descriptor.
This feature is available only for terminals, pseudoterminals,
sockets, and (since Linux 2.6) pipes and FIFOs.
See
fcntl(2)
for further details.
See also BUGS, below.

O_CLOEXEC (since Linux 2.6.23)

Enable the close-on-exec flag for the new file descriptor.
Specifying this flag permits a program to avoid additional
fcntl(2)
F_SETFD
operations to set the
FD_CLOEXEC
flag.

Note that the use of this flag is essential in some multithreaded programs,
because using a separate
fcntl(2)
F_SETFD
operation to set the
FD_CLOEXEC
flag does not suffice to avoid race conditions
where one thread opens a file descriptor and
attempts to set its close-on-exec flag using
fcntl(2)
at the same time as another thread does a
fork(2)
plus
execve(2).
Depending on the order of execution,
the race may lead to the file descriptor returned by
open()
being unintentionally leaked to the program executed by the child process
created by
fork(2).
(This kind of race is in principle possible for any system call
that creates a file descriptor whose close-on-exec flag should be set,
and various other Linux system calls provide an equivalent of the
O_CLOEXEC
flag to deal with this problem.)

O_CREAT

If the file does not exist, it will be created.

The owner (user ID) of the new file is set to the effective user ID
of the process.

The group ownership (group ID) of the new file is set either to
the effective group ID of the process (System V semantics)
or to the group ID of the parent directory (BSD semantics).
On Linux, the behavior depends on whether the
set-group-ID mode bit is set on the parent directory:
if that bit is set, then BSD semantics apply;
otherwise, System V semantics apply.
For some filesystems, the behavior also depends on the
bsdgroups
and
sysvgroups
mount options described in
mount(8)).

The
mode
argument specifies the file mode bits be applied when a new file is created.
This argument must be supplied when
O_CREAT
or
O_TMPFILE
is specified in
flags;
if neither
O_CREAT
nor
O_TMPFILE
is specified, then
mode
is ignored.
The effective mode is modified by the process's
umask
in the usual way: in the absence of a default ACL, the mode of the
created file is
(mode & ~umask).
Note that this mode applies only to future accesses of the
newly created file; the
open()
call that creates a read-only file may well return a read/write
file descriptor.

The following symbolic constants are provided for
mode:

S_IRWXU

00700 user (file owner) has read, write, and execute permission

S_IRUSR

00400 user has read permission

S_IWUSR

00200 user has write permission

S_IXUSR

00100 user has execute permission

S_IRWXG

00070 group has read, write, and execute permission

S_IRGRP

00040 group has read permission

S_IWGRP

00020 group has write permission

S_IXGRP

00010 group has execute permission

S_IRWXO

00007 others have read, write, and execute permission

S_IROTH

00004 others have read permission

S_IWOTH

00002 others have write permission

S_IXOTH

00001 others have execute permission

According to POSIX, the effect when other bits are set in
mode
is unspecified.
On Linux, the following bits are also honored in
mode:

Try to minimize cache effects of the I/O to and from this file.
In general this will degrade performance, but it is useful in
special situations, such as when applications do their own caching.
File I/O is done directly to/from user-space buffers.
The
O_DIRECT
flag on its own makes an effort to transfer data synchronously,
but does not give the guarantees of the
O_SYNC
flag that data and necessary metadata are transferred.
To guarantee synchronous I/O,
O_SYNC
must be used in addition to
O_DIRECT.
See NOTES below for further discussion.

A semantically similar (but deprecated) interface for block devices
is described in
raw(8).

O_DIRECTORY

If pathname is not a directory, cause the open to fail.
This flag was added in kernel version 2.1.126, to
avoid denial-of-service problems if
opendir(3)
is called on a
FIFO or tape device.

O_DSYNC

Write operations on the file will complete according to the requirements of
synchronized I/O
data
integrity completion.

By the time
write(2)
(and similar)
return, the output data
has been transferred to the underlying hardware,
along with any file metadata that would be required to retrieve that data
(i.e., as though each
write(2)
was followed by a call to
fdatasync(2)).
See NOTES below.

O_EXCL

Ensure that this call creates the file:
if this flag is specified in conjunction with
O_CREAT,
and
pathname
already exists, then
open()
will fail.

When these two flags are specified, symbolic links are not followed:
if
pathname
is a symbolic link, then
open()
fails regardless of where the symbolic link points to.

In general, the behavior of
O_EXCL
is undefined if it is used without
O_CREAT.
There is one exception: on Linux 2.6 and later,
O_EXCL
can be used without
O_CREAT
if
pathname
refers to a block device.
If the block device is in use by the system (e.g., mounted),
open()
fails with the error
EBUSY.

On NFS,
O_EXCL
is supported only when using NFSv3 or later on kernel 2.6 or later.
In NFS environments where
O_EXCL
support is not provided, programs that rely on it
for performing locking tasks will contain a race condition.
Portable programs that want to perform atomic file locking using a lockfile,
and need to avoid reliance on NFS support for
O_EXCL,
can create a unique file on
the same filesystem (e.g., incorporating hostname and PID), and use
link(2)
to make a link to the lockfile.
If
link(2)
returns 0, the lock is successful.
Otherwise, use
stat(2)
on the unique file to check if its link count has increased to 2,
in which case the lock is also successful.

O_LARGEFILE

(LFS)
Allow files whose sizes cannot be represented in an
off_t
(but can be represented in an
off64_t)
to be opened.
The
_LARGEFILE64_SOURCE
macro must be defined
(before including
any
header files)
in order to obtain this definition.
Setting the
_FILE_OFFSET_BITS
feature test macro to 64 (rather than using
O_LARGEFILE)
is the preferred
method of accessing large files on 32-bit systems (see
feature_test_macros(7)).

O_NOATIME (since Linux 2.6.8)

Do not update the file last access time
(st_atime
in the inode)
when the file is
read(2).

This flag can be employed only if one of the following conditions is true:

*

The effective UID of the process
matches the owner UID of the file.

*

The calling process has the
CAP_FOWNER
capability in its user namespace and
the owner UID of the file has a mapping in the namespace.

This flag is intended for use by indexing or backup programs,
where its use can significantly reduce the amount of disk activity.
This flag may not be effective on all filesystems.
One example is NFS, where the server maintains the access time.

O_NOCTTY

If
pathname
refers to a terminal device---see
tty(4)---it
will not become the process's controlling terminal even if the
process does not have one.

O_NOFOLLOW

If pathname is a symbolic link, then the open fails, with the error
ELOOP.
Symbolic links in earlier components of the pathname will still be
followed.
(Note that the
ELOOP
error that can occur in this case is indistinguishable from the case where
an open fails because there are too many symbolic links found
while resolving components in the prefix part of the pathname.)

This flag is a FreeBSD extension, which was added to Linux in version 2.1.126,
and has subsequently been standardized in POSIX.1-2008.

See also
O_PATH
below.

O_NONBLOCK or O_NDELAY

When possible, the file is opened in nonblocking mode.
Neither the
open()
nor any subsequent operations on the file descriptor which is
returned will cause the calling process to wait.

Note that this flag has no effect for regular files and block devices;
that is, I/O operations will (briefly) block when device activity
is required, regardless of whether
O_NONBLOCK
is set.
Since
O_NONBLOCK
semantics might eventually be implemented,
applications should not depend upon blocking behavior
when specifying this flag for regular files and block devices.

For the handling of FIFOs (named pipes), see also
fifo(7).
For a discussion of the effect of
O_NONBLOCK
in conjunction with mandatory file locks and with file leases, see
fcntl(2).

O_PATH (since Linux 2.6.39)

Obtain a file descriptor that can be used for two purposes:
to indicate a location in the filesystem tree and
to perform operations that act purely at the file descriptor level.
The file itself is not opened, and other file operations (e.g.,
read(2),
write(2),
fchmod(2),
fchown(2),
fgetxattr(2),
ioctl(2),
mmap(2))
fail with the error
EBADF.

The following operations
can
be performed on the resulting file descriptor:

Retrieving open file status flags using the
fcntl(2)
F_GETFL
operation: the returned flags will include the bit
O_PATH.

*

Passing the file descriptor as the
dirfd
argument of
openat()
and the other "*at()" system calls.
This includes
linkat(2)
with
AT_EMPTY_PATH
(or via procfs using
AT_SYMLINK_FOLLOW)
even if the file is not a directory.

*

Passing the file descriptor to another process via a UNIX domain socket
(see
SCM_RIGHTS
in
unix(7)).

When
O_PATH
is specified in
flags,
flag bits other than
O_CLOEXEC,
O_DIRECTORY,
and
O_NOFOLLOW
are ignored.

If
pathname
is a symbolic link and the
O_NOFOLLOW
flag is also specified,
then the call returns a file descriptor referring to the symbolic link.
This file descriptor can be used as the
dirfd
argument in calls to
fchownat(2),
fstatat(2),
linkat(2),
and
readlinkat(2)
with an empty pathname to have the calls operate on the symbolic link.

If
pathname
refers to an automount point that has not yet been triggered, so no
other filesystem is mounted on it, then the call returns a file
descriptor referring to the automount directory without triggering a mount.
fstatfs(2)
can then be used to determine if it is, in fact, an untriggered
automount point
(.f_type == AUTOFS_SUPER_MAGIC).

O_SYNC

Write operations on the file will complete according to the requirements of
synchronized I/O
file
integrity completion
(by contrast with the
synchronized I/O
data
integrity completion
provided by
O_DSYNC.)

By the time
write(2)
(and similar)
return, the output data and associated file metadata
have been transferred to the underlying hardware
(i.e., as though each
write(2)
was followed by a call to
fsync(2)).
See NOTES below.

O_TMPFILE (since Linux 3.11)

Create an unnamed temporary file.
The
pathname
argument specifies a directory;
an unnamed inode will be created in that directory's filesystem.
Anything written to the resulting file will be lost when
the last file descriptor is closed, unless the file is given a name.

O_TMPFILE
must be specified with one of
O_RDWR
or
O_WRONLY
and, optionally,
O_EXCL.
If
O_EXCL
is not specified, then
linkat(2)
can be used to link the temporary file into the filesystem, making it
permanent, using code like the following:

In this case,
the
open()
mode
argument determines the file permission mode, as with
O_CREAT.

Specifying
O_EXCL
in conjunction with
O_TMPFILE
prevents a temporary file from being linked into the filesystem
in the above manner.
(Note that the meaning of
O_EXCL
in this case is different from the meaning of
O_EXCL
otherwise.)

There are two main use cases for
O_TMPFILE:

*

Improved
tmpfile(3)
functionality: race-free creation of temporary files that
(1) are automatically deleted when closed;
(2) can never be reached via any pathname;
(3) are not subject to symlink attacks; and
(4) do not require the caller to devise unique names.

*

Creating a file that is initially invisible, which is then populated
with data and adjusted to have appropriate filesystem attributes
(fchown(2),
fchmod(2),
fsetxattr(2),
etc.)
before being atomically linked into the filesystem
in a fully formed state (using
linkat(2)
as described above).

O_TMPFILE
requires support by the underlying filesystem;
only a subset of Linux filesystems provide that support.
In the initial implementation, support was provided in
the ext2, ext3, ext4, UDF, Minix, and shmem filesystems.
Support for other filesystems has subsequently been added as follows:
XFS (Linux 3.15);
Btrfs (Linux 3.16);
F2FS (Linux 3.16);
and ubifs (Linux 4.9)

O_TRUNC

If the file already exists and is a regular file and the access mode allows
writing (i.e., is
O_RDWR
or
O_WRONLY)
it will be truncated to length 0.
If the file is a FIFO or terminal device file, the
O_TRUNC
flag is ignored.
Otherwise, the effect of
O_TRUNC
is unspecified.

creat()

A call to
creat()
is equivalent to calling
open()
with
flags
equal to
O_CREAT|O_WRONLY|O_TRUNC.

openat()

The
openat()
system call operates in exactly the same way as
open(),
except for the differences described here.

If the pathname given in
pathname
is relative, then it is interpreted relative to the directory
referred to by the file descriptor
dirfd
(rather than relative to the current working directory of
the calling process, as is done by
open()
for a relative pathname).

If
pathname
is relative and
dirfd
is the special value
AT_FDCWD,
then
pathname
is interpreted relative to the current working
directory of the calling process (like
open()).

If
pathname
is absolute, then
dirfd
is ignored.

RETURN VALUE

open(),
openat(),
and
creat()
return the new file descriptor, or -1 if an error occurred
(in which case,
errno
is set appropriately).

ERRORS

open(),
openat(),
and
creat()
can fail with the following errors:

EACCES

The requested access to the file is not allowed, or search permission
is denied for one of the directories in the path prefix of
pathname,
or the file did not exist yet and write access to the parent directory
is not allowed.
(See also
path_resolution(7).)

EDQUOT

Where
O_CREAT
is specified, the file does not exist, and the user's quota of disk
blocks or inodes on the filesystem has been exhausted.

EEXIST

pathname
already exists and
O_CREAT and O_EXCL
were used.

EFAULT

pathname
points outside your accessible address space.

EFBIG

See
EOVERFLOW.

EINTR

While blocked waiting to complete an open of a slow device
(e.g., a FIFO; see
fifo(7)),
the call was interrupted by a signal handler; see
signal(7).

EINVAL

The filesystem does not support the
O_DIRECT
flag.
See
NOTES
for more information.

EINVAL

Invalid value in
flags.

EINVAL

O_TMPFILE
was specified in
flags,
but neither
O_WRONLY
nor
O_RDWR
was specified.

EISDIR

pathname
refers to a directory and the access requested involved writing
(that is,
O_WRONLY
or
O_RDWR
is set).

EISDIR

pathname
refers to an existing directory,
O_TMPFILE
and one of
O_WRONLY
or
O_RDWR
were specified in
flags,
but this kernel version does not provide the
O_TMPFILE
functionality.

ELOOP

Too many symbolic links were encountered in resolving
pathname.

ELOOP

pathname
was a symbolic link, and
flags
specified
O_NOFOLLOW
but not
O_PATH.

EMFILE

The per-process limit on the number of open file descriptors has been reached
(see the description of
RLIMIT_NOFILE
in
getrlimit(2)).

ENAMETOOLONG

pathname
was too long.

ENFILE

The system-wide limit on the total number of open files has been reached.

ENODEV

pathname
refers to a device special file and no corresponding device exists.
(This is a Linux kernel bug; in this situation
ENXIO
must be returned.)

ENOENT

O_CREAT
is not set and the named file does not exist.
Or, a directory component in
pathname
does not exist or is a dangling symbolic link.

ENOENT

pathname
refers to a nonexistent directory,
O_TMPFILE
and one of
O_WRONLY
or
O_RDWR
were specified in
flags,
but this kernel version does not provide the
O_TMPFILE
functionality.

ENOMEM

The named file is a FIFO,
but memory for the FIFO buffer can't be allocated because
the per-user hard limit on memory allocation for pipes has been reached
and the caller is not privileged; see
pipe(7).

ENOMEM

Insufficient kernel memory was available.

ENOSPC

pathname
was to be created but the device containing
pathname
has no room for the new file.

ENOTDIR

A component used as a directory in
pathname
is not, in fact, a directory, or O_DIRECTORY was specified and
pathname
was not a directory.

ENXIO

O_NONBLOCK | O_WRONLY
is set, the named file is a FIFO, and
no process has the FIFO open for reading.

ENXIO

The file is a device special file and no corresponding device exists.

EOPNOTSUPP

The filesystem containing
pathname
does not support
O_TMPFILE.

EOVERFLOW

pathname
refers to a regular file that is too large to be opened.
The usual scenario here is that an application compiled
on a 32-bit platform without
-D_FILE_OFFSET_BITS=64
tried to open a file whose size exceeds
(1<<31)-1
bytes;
see also
O_LARGEFILE
above.
This is the error specified by POSIX.1;
in kernels before 2.6.24, Linux gave the error
EFBIG
for this case.

EPERM

The
O_NOATIME
flag was specified, but the effective user ID of the caller
did not match the owner of the file and the caller was not privileged.

pathname
refers to a file on a read-only filesystem and write access was
requested.

ETXTBSY

pathname
refers to an executable image which is currently being executed and
write access was requested.

EWOULDBLOCK

The
O_NONBLOCK
flag was specified, and an incompatible lease was held on the file
(see
fcntl(2)).

The following additional errors can occur for
openat():

EBADF

dirfd
is not a valid file descriptor.

ENOTDIR

pathname
is a relative pathname and
dirfd
is a file descriptor referring to a file other than a directory.

VERSIONS

openat()
was added to Linux in kernel 2.6.16;
library support was added to glibc in version 2.4.

CONFORMING TO

open(),
creat()
SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.

openat():
POSIX.1-2008.

The
O_DIRECT,
O_NOATIME,
O_PATH,
and
O_TMPFILE
flags are Linux-specific.
One must define
_GNU_SOURCE
to obtain their definitions.

The
O_CLOEXEC,
O_DIRECTORY,
and
O_NOFOLLOW
flags are not specified in POSIX.1-2001,
but are specified in POSIX.1-2008.
Since glibc 2.12, one can obtain their definitions by defining either
_POSIX_C_SOURCE
with a value greater than or equal to 200809L or
_XOPEN_SOURCE
with a value greater than or equal to 700.
In glibc 2.11 and earlier, one obtains the definitions by defining
_GNU_SOURCE.

As noted in
feature_test_macros(7),
feature test macros such as
_POSIX_C_SOURCE,
_XOPEN_SOURCE,
and
_GNU_SOURCE
must be defined before including
any
header files.

NOTES

Under Linux, the
O_NONBLOCK
flag indicates that one wants to open
but does not necessarily have the intention to read or write.
This is typically used to open devices in order to get a file descriptor
for use with
ioctl(2).

The (undefined) effect of
O_RDONLY | O_TRUNC
varies among implementations.
On many systems the file is actually truncated.

Note that
open()
can open device special files, but
creat()
cannot create them; use
mknod(2)
instead.

If the file is newly created, its
st_atime,
st_ctime,
st_mtime
fields
(respectively, time of last access, time of last status change, and
time of last modification; see
stat(2))
are set
to the current time, and so are the
st_ctime
and
st_mtime
fields of the
parent directory.
Otherwise, if the file is modified because of the
O_TRUNC
flag, its
st_ctime
and
st_mtime
fields are set to the current time.

The files in the
/proc/[pid]/fd
directory show the open file descriptors of the process with the PID
pid.
The files in the
/proc/[pid]/fdinfo
directory show even more information about these files descriptors.
See
proc(5)
for further details of both of these directories.

Open file descriptions

The term open file description is the one used by POSIX to refer to the
entries in the system-wide table of open files.
In other contexts, this object is
variously also called an "open file object",
a "file handle", an "open file table entry",
or---in kernel-developer parlance---a
struct file.

When a file descriptor is duplicated (using
dup(2)
or similar),
the duplicate refers to the same open file description
as the original file descriptor,
and the two file descriptors consequently share
the file offset and file status flags.
Such sharing can also occur between processes:
a child process created via
fork(2)
inherits duplicates of its parent's file descriptors,
and those duplicates refer to the same open file descriptions.

Each
open()
of a file creates a new open file description;
thus, there may be multiple open file descriptions
corresponding to a file inode.

On Linux, one can use the
kcmp(2)
KCMP_FILE
operation to test whether two file descriptors
(in the same process or in two different processes)
refer to the same open file description.

Synchronized I/O

The POSIX.1-2008 "synchronized I/O" option
specifies different variants of synchronized I/O,
and specifies the
open()
flags
O_SYNC,
O_DSYNC,
and
O_RSYNC
for controlling the behavior.
Regardless of whether an implementation supports this option,
it must at least support the use of
O_SYNC
for regular files.

Linux implements
O_SYNC
and
O_DSYNC,
but not
O_RSYNC.
(Somewhat incorrectly, glibc defines
O_RSYNC
to have the same value as
O_SYNC.)

O_SYNC
provides synchronized I/O
file
integrity completion,
meaning write operations will flush data and all associated metadata
to the underlying hardware.
O_DSYNC
provides synchronized I/O
data
integrity completion,
meaning write operations will flush data
to the underlying hardware,
but will only flush metadata updates that are required
to allow a subsequent read operation to complete successfully.
Data integrity completion can reduce the number of disk operations
that are required for applications that don't need the guarantees
of file integrity completion.

To understand the difference between the two types of completion,
consider two pieces of file metadata:
the file last modification timestamp
(st_mtime)
and the file length.
All write operations will update the last file modification timestamp,
but only writes that add data to the end of the
file will change the file length.
The last modification timestamp is not needed to ensure that
a read completes successfully, but the file length is.
Thus,
O_DSYNC
would only guarantee to flush updates to the file length metadata
(whereas
O_SYNC
would also always flush the last modification timestamp metadata).

Before Linux 2.6.33, Linux implemented only the
O_SYNC
flag for
open().
However, when that flag was specified,
most filesystems actually provided the equivalent of synchronized I/O
data
integrity completion (i.e.,
O_SYNC
was actually implemented as the equivalent of
O_DSYNC).

Since Linux 2.6.33, proper
O_SYNC
support is provided.
However, to ensure backward binary compatibility,
O_DSYNC
was defined with the same value as the historical
O_SYNC,
and
O_SYNC
was defined as a new (two-bit) flag value that includes the
O_DSYNC
flag value.
This ensures that applications compiled against
new headers get at least
O_DSYNC
semantics on pre-2.6.33 kernels.

NFS

There are many infelicities in the protocol underlying NFS, affecting
amongst others
O_SYNC and O_NDELAY.

On NFS filesystems with UID mapping enabled,
open()
may
return a file descriptor but, for example,
read(2)
requests are denied
with EACCES.
This is because the client performs
open()
by checking the
permissions, but UID mapping is performed by the server upon
read and write requests.

FIFOs

Opening the read or write end of a FIFO blocks until the other
end is also opened (by another process or thread).
See
fifo(7)
for further details.

File access mode

Unlike the other values that can be specified in
flags,
the
access mode
values
O_RDONLY, O_WRONLY, and O_RDWR
do not specify individual bits.
Rather, they define the low order two bits of
flags,
and are defined respectively as 0, 1, and 2.
In other words, the combination
O_RDONLY | O_WRONLY
is a logical error, and certainly does not have the same meaning as
O_RDWR.

Linux reserves the special, nonstandard access mode 3 (binary 11) in
flags
to mean:
check for read and write permission on the file and return a file descriptor
that can't be used for reading or writing.
This nonstandard access mode is used by some Linux drivers to return a
file descriptor that is to be used only for device-specific
ioctl(2)
operations.

First,
openat()
allows an application to avoid race conditions that could
occur when using
open()
to open files in directories other than the current working directory.
These race conditions result from the fact that some component
of the directory prefix given to
open()
could be changed in parallel with the call to
open().
Suppose, for example, that we wish to create the file
path/to/xxx.dep
if the file
path/to/xxx
exists.
The problem is that between the existence check and the file creation step,
path
or
to
(which might be symbolic links)
could be modified to point to a different location.
Such races can be avoided by
opening a file descriptor for the target directory,
and then specifying that file descriptor as the
dirfd
argument of (say)
fstatat(2)
and
openat().
The use of the
dirfd
file descriptor also has other benefits:

*

the file descriptor is a stable reference to the directory,
even if the directory is renamed; and

*

the open file descriptor prevents the underlying filesystem from
being dismounted,
just as when a process has a current working directory on a filesystem.

Second,
openat()
allows the implementation of a per-thread "current working
directory", via file descriptor(s) maintained by the application.
(This functionality can also be obtained by tricks based
on the use of
/proc/self/fd/dirfd,
but less efficiently.)

O_DIRECT

The
O_DIRECT
flag may impose alignment restrictions on the length and address
of user-space buffers and the file offset of I/Os.
In Linux alignment
restrictions vary by filesystem and kernel version and might be
absent entirely.
However there is currently no filesystem-independent
interface for an application to discover these restrictions for a given
file or filesystem.
Some filesystems provide their own interfaces
for doing so, for example the
XFS_IOC_DIOINFO
operation in
xfsctl(3).

Under Linux 2.4, transfer sizes, and the alignment of the user buffer
and the file offset must all be multiples of the logical block size
of the filesystem.
Since Linux 2.6.0, alignment to the logical block size of the
underlying storage (typically 512 bytes) suffices.
The logical block size can be determined using the
ioctl(2)
BLKSSZGET
operation or from the shell using the command:

blockdev --getss

O_DIRECT
I/Os should never be run concurrently with the
fork(2)
system call,
if the memory buffer is a private mapping
(i.e., any mapping created with the
mmap(2)
MAP_PRIVATE
flag;
this includes memory allocated on the heap and statically allocated buffers).
Any such I/Os, whether submitted via an asynchronous I/O interface or from
another thread in the process,
should be completed before
fork(2)
is called.
Failure to do so can result in data corruption and undefined behavior in
parent and child processes.
This restriction does not apply when the memory buffer for the
O_DIRECT
I/Os was created using
shmat(2)
or
mmap(2)
with the
MAP_SHARED
flag.
Nor does this restriction apply when the memory buffer has been advised as
MADV_DONTFORK
with
madvise(2),
ensuring that it will not be available
to the child after
fork(2).

The
O_DIRECT
flag was introduced in SGI IRIX, where it has alignment
restrictions similar to those of Linux 2.4.
IRIX has also a
fcntl(2)
call to query appropriate alignments, and sizes.
FreeBSD 4.x introduced
a flag of the same name, but without alignment restrictions.

O_DIRECT
support was added under Linux in kernel version 2.4.10.
Older Linux kernels simply ignore this flag.
Some filesystems may not implement the flag and
open()
will fail with
EINVAL
if it is used.

Applications should avoid mixing
O_DIRECT
and normal I/O to the same file,
and especially to overlapping byte regions in the same file.
Even when the filesystem correctly handles the coherency issues in
this situation, overall I/O throughput is likely to be slower than
using either mode alone.
Likewise, applications should avoid mixing
mmap(2)
of files with direct I/O to the same files.

The behavior of
O_DIRECT
with NFS will differ from local filesystems.
Older kernels, or
kernels configured in certain ways, may not support this combination.
The NFS protocol does not support passing the flag to the server, so
O_DIRECT
I/O will bypass the page cache only on the client; the server may
still cache the I/O.
The client asks the server to make the I/O
synchronous to preserve the synchronous semantics of
O_DIRECT.
Some servers will perform poorly under these circumstances, especially
if the I/O size is small.
Some servers may also be configured to
lie to clients about the I/O having reached stable storage; this
will avoid the performance penalty at some risk to data integrity
in the event of server power failure.
The Linux NFS client places no alignment restrictions on
O_DIRECT
I/O.

In summary,
O_DIRECT
is a potentially powerful tool that should be used with caution.
It is recommended that applications treat use of
O_DIRECT
as a performance option which is disabled by default.

"The thing that has always disturbed me about O_DIRECT is that the whole
interface is just stupid, and was probably designed by a deranged monkey
on some serious mind-controlling substances."---Linus

BUGS

Currently, it is not possible to enable signal-driven
I/O by specifying
O_ASYNC
when calling
open();
use
fcntl(2)
to enable this flag.

One must check for two different error codes,
EISDIR
and
ENOENT,
when trying to determine whether the kernel supports
O_TMPFILE
functionality.

When both
O_CREAT
and
O_DIRECTORY
are specified in
flags
and the file specified by
pathname
does not exist,
open()
will create a regular file (i.e.,
O_DIRECTORY
is ignored).

COLOPHON

This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.